Synthetic tanning agents are conventionally prepared by the condensation of naphthalene/phenol sulphonic acid or urea with formaldehyde as reported by Sharphouse (Leather Technicians Handbook, Vernon Lock Ltd., London, 1983).
As reported by Stille and Campbell (High Polymer, 27, 14, 1972), the phenol-formaldehyde condensed product, which is essentially a low molecular weight resinous material, has always a possibility of eliminating unreacted phenol or formaldehyde, thereby posing a risk of adding to toxicity.
Denzinger et al. (U.S. Pat. No. 5,425,784) discloses the preparation of water-soluble graft polymers of monosaccharides, oligosaccharides, polysaccharides and derivatives with crotonic acid, maleic acid, maleic anhydride, fumaric acid, citraconic acid, citraconic anhydride, C1–C8-alkyl- and hydroxyalkyl esters of acrylic acid, methacrylic acid or crotonic acid, mono- and di-C1–C8-alkyl and -hydroxyalkyl esters of maleic acid, fumaric acid and citraconic acid, acrylamide, methacrylamide, methacrolein, acrylamido-methylphropanesulfonic acid, N-vinylimidazole and mixtures. These compounds can be used as tanning agents for the self-tanning, pretanning, simultaneous tanning and retanning of pelts and skins. However, these polymers do not produce leathers having fine grain, good fullness, affordable heat stability and uniform color.
Wehling et al (U.S. Pat. No. 5,340,365) discloses the use of naphthalene sulphonic acid condensate in the production of chrome leathers. They have reported that this condensation product can be used for pretreatment of delimed and bated pelts. Although the properties of the chrome tanned leathers produced using naphthalene sulphonic acid condensate do not differ from those of conventionally chrome tanned leathers, the condensed product was prepared using formaldehyde. The condensed product may undergo photo-oxidation and release formaldehyde which may pose occupational hazards (Suspected carcinogens: A sub life of the National Institute for Occupational Safety and Health (NIOSH) substances list, 2nd ed. Washington D.C.: U.S. Dept of Health, Education and Welfare NIOSH, 1976). Moreover, the developed chrome tanning method is associated with release of chromium in the effluent beyond the stipulated limit (2 ppm as Cr) as reported by Buljan (World Leather, November, 65, 1996).
Although the phenols and phenolic acid resins based on formaldehyde condensation have exhibited a certain amount of freedom of choice with respect to degree of crosslinking, molecular weight, particle size, etc. there are several limitations related to the use of these polymeric condensates. These agents considerably lighten the color of the leather. Addition of large amounts of the agents lead to a rubber-like handle of the leather. The tanned leathers obtained using these agents undergo photo-oxidation due to the presence of active methylene bridges. Slow release of formaldehyde from the tanned leathers may also pose hazardous environmental risks.
Current chrome tanning methods pose severe problems arising from associated environmental problems. Industry has taken great pains to improve the chromium exhaustion. Prentiss and Sigafoos reported that the use of poly acrylic and poly methacrylic acids in chrome tanning improves the chrome exhaustion (Journal of American Leather Chemists Association, 70, 481, 1975). In addition, Lakshminarayana et al. disclosed an acrylic acid based syntan as retanning agent for chrome tanned leathers, which is prepared by the graft co-polymerization of acrylic ester and methacrylic acid onto a mixture of sulfated fish oil and alginic acid using free radical polymerization (Journal of the American Leather Chemists Association, 85, 425, 1990). However, the major limitation associated with the acrylic system is that it leads to poor light fastness and heat resistance. Moreover, the acrylic syntans, which are generally attached to the protein through weak linkage, result in either low exhaustion in the processing bath or more leaching out during subsequent wet operations.
Christner et al (U.S. Pat. No. 4,961,750) have reported the preparation of thio-containing chromium polyacrylate tanning agents by complexing the chromium ion and an acrylate polymer together with a heterocycle containing nitrogen, oxygen, or sulfur atom, having a molecular weight between 200 and 30,000, which can be offered to pickled skins or hides. The main disadvantage of the tanning agent disclosed by Christner et al. is the slow release of oxides of sulphur, which are carcinogenic.
The above limitations have prompted the possibility of incorporating metal ion onto a polymeric matrix to form a complex for use as tanning salt. Since formaldehyde produces a known toxic hazard, attempts have been made to prepare a formaldehyde-free synthetic tanning agent (Suresh et al. Journal of Cleaner Production, 9, 483, 2001). Thanikaivelan et al. demonstrated the use of a synthetic tanning agent based on complexation of chromium with acrylic polymer and sulfonated aromatic condensates in the presence of organic ligands for self-tanning of delimed pelts and retanning applications (Journal of American Leather Chemists Association, 97, 127, 2002). The disclosed product resulted in improved exhaustion of chromium when applied on delimed pelts and reduced the total dissolved solids in the effluent. The major limiting feature associated with the syntan taught by Thanikaivelan et al. involves the use of chromium salt for complexation, which is toxic due to the presence of chrome.
Thus, there is a need for the development of a synthetic tanning agent that does not produce toxic effects. The tanning agent must be eco-friendly and safe for workers. Furthermore, the tanning agent should minimize the use of toxic substances, such as formaldehyde and chromium.